The objectives of this research are to understand the mechanism by which expression of the endogenous viral genes of the chicken is regulated, to determine whether these genes play a role in ontogeny, and to assess the involvement, if any, of these genes in oncogenesis and disease. Using restriction endonuclease cleavage of chicken DNA and the blotting technique of Southern, we have identified ten sites of residence of endogenous viral genes in the chicken genome. In mating experiments, the viral information at each of the ten sites can be seen to segregate as an individual genetic locus. Six of the loci are each associated with a specific phenotype of viral gene expression such as V plus, gs plus chf plus, and gs minus chf plus. Proposed experiments include: 1) Analysis of the segregation of specific phenotypes and of individual loci using the progeny of specific matings. 2) Establishment of lines of chickens with no ALV related endogenous viral sequences and with specific complements of viral loci. 3) Using cDNA proves for specific regions of the viral genome to obtain information as to the nature of the viral genes at each of the ten loci.